High quality steel products such as spur gears for aerospace applications require that the gear surface making contact with other gears be hardened to minimize wear, while the interior portion of the gear must remain unhardened in order to prevent the gear from becoming brittle, shock-susceptible, and subject to breakage. The typical industrial process for manufacturing high quality gears requires either case carburizing and hardening, or induction hardening, of the gear teeth to a specified contour, case depth, and hardness.
Carburizing introduces carbon into the surface layer of a low-carbon steel by heating the part in a furnace while it is in contact with a carbonaceous material. The carbon diffuses into the steel from the surface and converts the outer layer of the part into high-carbon steel. The part is then removed from the furnace, allowed to cool, and is heat-treated be being brought to a high temperature above the transformation point and quickly quenched, transforming the high-carbon surface layer into a hard case containing martensite, while leaving the low-carbon core tough and shock-resistant. Quenching involves a rapid cooling of the heated surfaces either conventionally by a gas or a liquid, or by the heat sink effect of the part's mass (not possible where the part is heated in a furnace).
Carburizing requires selective masking of the part, as well as subsequent chemical mask removal, to prevent surface portions of the part which must remain non-hardened from becoming hardened in the carburizing process. The quenching step produces distortion in the part, which will then invariably require a final grinding operation to correct the distortion, particularly in gears destined for use in aerospace applications, which are required to be of extremely high quality and have critical tolerances.
In the manufacturing of such high quality gears, quenching dies may be used to minimize distortion during the quenching operation. The gear being hardened is heated above the transformation temperature, and is placed into a quenching die fitting the part perfectly. The quenching operation is then performed, and the part may be removed from the quenching die.
Thus, it can be seen that the carburizing method of hardening is both energy and labor intensive, and is therefore quite expensive. In addition, the carburizing technique is quite time-consuming and requires a large amount of equipment, including a furnace, quenching dies must be custom made for each part being manufactured, masking equipment, and regrinding equipment.
An alternative to carburizing is induction hardening, where the part to be hardened is placed inside a coil through which a rapidly alternating current is flowing. Heat is rapidly generated within localized portions of the part by electromagnetic induction, which the depth of the case being controlled by the frequency of the current in the coil. The part is then quenched, and induction hardening thus also presents the problem of distortion in the part which will subsequently require final regrinding operations. As such, induction hardening is also expensive and time-consuming.
Due to their unique properties, industiral lasers have shown great promise in selective rapid heating of a surface. The surface is generally prepared by applying an absorptive coating to the surface to be heated, aiding in energy transfer from the laser beam to heat energy within the part. By using a laser to quickly heat a surface, conventional quenching by a gas or a liquid is unnecessary since only the shallow surface area itself is heated. The part will actually self-quench, due to the extremely high heat differential between the shallow surface layer heated by the laser and the bulk of the part being processed. This is in sharp contrast to carburizing or induction heating, where the part must be heated in one operation, and then is required to be quickly quenched by a gas or a liquid.
Attempts have been made in the past to use industrial lasers for surface heat treatment of parts such as gears, and two such attempts are described in U.S. Pat. Nos. 4,250,372 and 4,250,374, both to Tani, The '374 patent describes the technique of gear hardening using a single beam, and the '372 patent describes a technique using two or more beams to obtain more even heating of the gear tooth areas to be hardened.
Both of these patents, however, have one thing in common which renders them largely impractical--they seek to harden a gear by hardening a V-shaped area including the flank or side of one gear tooth, the flank of an adjacent gear tooth, and the root area between the two gear teeth at one time, and then move on to an adjacent V-shaped area. The problem with such an approach is that when one flank of a gear tooth is hardened in one operation, and the opposite flank of the gear tooth is hardened in a second operation, sufficient heat is generated in the gear tooth when the second flank is hardened to substantially diminish the hardness in a portion of the first flank. This problem, which presents itself in the hardening of all but very coarse gears, is referred to as back-temper, since it heats and softens a surface which has already been hardened.
Therefore, it can be seen that although the '372 and the '374 patents do provide desireable alternatives to carburizing, the alternative presented is impractical on all but very coarse gears. Since a great number of the high quality aerospace spur gears manufactured are considerably smaller than the size which can be manufactured by the process described in the '372 and the '374 patents, it can seen that these patents simply do not present a viable, broadranging alternative to carburizing and hardening of gears.
Additionally, the beam splitting device shown in the '372 patent has been found to be prone to differences in the two beam intensities caused by extremely small dimensional errors, causing inconsistent heating of the areas heated by the two beams. The use of two discrete laser sources is too expensive and requires too large an installation to be practical. Thus, it can be seen that if lasers are to be used for surface hardening of parts such as gears, a new method and/or apparatus for hardening gears which eliminates the back temper problem must be achieved.